Floristic patterns, ecological gradients and biodiversity in the composite channels (Central Alps, Italy)

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Abstract

Composite channels in the alpine region are heterogeneous environments resulting from a variety of erosive and depositional processes. They can have different ecological functions: habitats for plant species, conduits, filters, sources and sinks. They can contain sparse, intermittent vegetation cover, and are frequently free of vegetation except along the banks and levees. We hypothesised that this disturbed and fragmented landscape unit encourages biodiversity with characteristic plant species adapted to survive in the channel niches.

In this study, which was carried out mostly in the subalpine belt of the Adamello-Presanella Group, we aimed to characterize floristic and biodiversity patterns with respect to the dominant channel gradients by means of a canonical correspondence analysis (CCA) ordination approach and an analysis of diversity indexes. The frequency table and CCA results showed considerable floristic heterogeneity within the composite channels mainly as a function of nutrient availability and altitude. Different gradients of biodiversity (alpha, beta and gamma diversity) depended on the geomorphological disturbances (alpha and gamma) and environmental heterogeneity across the channel subunits.

Geomorphological processes seem to play key roles in the dispersal of seeds, spores and propagules and in the colonization, establishment and spread of plants in this complex environment. The composite channels could play an important refuge–habitat function for microthermic species during the widely accepted current trend of global warming.

Introduction

In recent years, interest in river and stream corridors and riparian habitats has increased, especially in regards to variations in vegetation structure and dynamics and in relation to differences in biodiversity gradients (Dynesius et al., 2004; Lyon and Gross, 2005; Nilsson and Svedmark, 2002). These environments have five principal functions: habitats for plant species, conduits, filters, sources and sinks (Butler, 2001). River and stream corridors produce heterogeneity by introducing strong ecological gradients that extend vertically, laterally and longitudinally with regards to natural disturbances and time (van Coller et al., 2000). In particular, disturbances deriving from geomorphological processes, at various spatial and temporal scales, seem to control species distribution (Ward et al., 2002). Moreover, environmental heterogeneity and disturbances influence plant species diversity patterns (Pollock et al., 1998).

There is a general agreement that climate changes could play a major role in modifying species distribution in mountain areas controlling disturbance effects upon vegetation. Azonal plant communities growing within channels seem to be less sensitive to climate change than zonal communities, since the major limiting factors of azonal assemblages are non-climatic in character (Kienast et al., 1998). On the other hand climatic warming may accelerate the invasion of alien plant species both in plain and mountain areas (Alexander et al., 2009; Hansen and Clevenger, 2005).

At the microscale, the spatial distribution of plants is mainly due to variations in the retention of nutrients (Osborne and Kovacic, 1993), in microclimatic conditions within streams (Grey and Eddington, 1969), or in the sedimentological and textural characteristics of deposits (Naiman and Décamps, 1997). Most studies that addressed the bio-ecological processes within rivers and corridors have been carried out in large- or medium-sized watercourses, generally in alluvial plains (Ward, 1998), in urbanized landscapes (Oneal and Rotenberry, 2008) and in arid environments (Tiegs et al., 2005); only a few authors have investigated arctic rivers or proglacial rivers (Gould and Walker, 1999; Gurnell et al., 1999). Furthermore, few specific studies have been conducted on floristic and diversity patterns within channels in the alpine region. Disturbance factors (fire, flooding, snow avalanches, etc.) play a major role in determining species distribution patterns and increase in diversity (Lenssen et al., 2004). In particular habitat diversity and species richness are considerably influenced by disturbances within avalanche tracks (Rixen et al., 2007). Snow avalanches are ecological agents that result in distinctive vegetation patterns in mountain environments (Stoeckli et al., 2005).

In our opinion, the floristic and biodiversity gradients within river/stream corridors are emphasised in the alpine regions, where typical species exist in high-quality environments. Along the valley slopes, geomorphic processes, such as running water, debris flow and snow avalanches (and, in part, also debris/rock fall), are extremely active and concentrated in the “central channel” (i.e. composite channel), a unit of the “composite alpine debris cones” (Baroni et al., 2007). Such processes, along with altitudinal gradients, seem to influence the dispersal (of seeds, spores and propagules), colonization, spread and establishment of these species in the alpine corridors.

In this study, we hypothesised that this disturbed and fragmented landscape unit favours floristic heterogeneity and biodiversity with characteristic plant species adapted to survive in the channel niches under selective environmental conditions. Our specific objectives were: (a) to assess and characterize the floristic patterns according to vertical, lateral and longitudinal ecological gradients and environmental indexes (i.e. Landolt indexes) within the composite channels; (b) to evaluate the spatial distribution of plant diversity in this environment in relation to geomorphic disturbances and (c) to stress the role of the channels as a means of dispersal/plant migration in the alpine environment.

Section snippets

Study area

The composite channels investigated are located in the Adamello-Presanella Group (Central Alps, Italy; Fig. 1A). The landscape in the highest elevation belt (above about 2500 m a.s.l.) is characterized by active glacial and periglacial landforms. The summit area of the group holds the largest glacier of the Italian Alps (Adamello Glacier, about 1813 ha in 1983, SGL 1992), as well as about 90 other minor glacial bodies.

The geological structure of the Adamello Group is mostly characterized by a

Floristic patterns

In total, 126 species were found within the channels. Most of them were occasional and were found only once (39 species) or twice (13 species). The most frequent species along the channels, with a presence in more than 50 plots (out of a total of 245 plots with plants), were Agrostis schraderana (37.5%), Alnus viridis (35.1%), Luzula alpino-pilosa (24.1%), Rumex scutatus (22.5%) and Poa alpina (21.6%).

Most of the empty plots were surveyed in the channel bed units (particularly in the upper

Discussion

Floristic analyses showed that the channels were strongly dominated by few species: A. schraderana, A. viridis, L. alpino-pilosa, R. scutatus and P. alpina. In the whole channel system, A. schraderana played the role of the dominant/ubiquitous species with weak relationships with the geomorphologic disturbance: the species was not found only in the CB_u, which is the most disturbed subunit.

Nevertheless, our results (both frequency table and CCA) evidenced that the composite channels in the

Acknowledgments

This study was funded by Pisa University, Milano-Bicocca University and the Museo Civico di Scienze Naturali di Brescia. The authors are grateful to Brigitta Erschbamer (Institute of Botany, University of Innsbruck) for helpful critical comments to the manuscript.

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